1. Field of the Invention
The present invention relates to a magnetic field sensor to measure a magnetic field from a magnetic field source.
2. Description of the Related Art
As a conventional magnetic field sensor, a shielded loop-type magnetic field sensor fabricated by processing a semi-rigid coaxial cable is used. The conventional magnetic sensor is excellent in electric shield characteristics. As shown in FIG. 5, in the conventional one, the semi-rigid coaxial cable 31 is configured so as to form a loop. A loop terminating point 32 of the coaxial cable 31 is connected to a loop starting point 37 of the coaxial cable 31 to establish a short circuit or the loop is terminated to be non-reflective. In the lowest portion of the loop in FIG. 5, a space 34 is formed with only a central conductor 33 left therein. At a portion 35 on the loop terminating point 32 of the space 34, a short circuit is established between the central conductor 33 and an external conductor. When the shielded loop magnetic sensor is put in a magnetic field, according to Faraday's law, a voltage can be obtained as a detected magnetic field output depending on a change with time in the magnetic field which links on a loop face 36.
When a magnetic field is to be measured by a source generating a magnetic field to be measured by a conventional magnetic field sensor, if there is another magnetic field generating source or a surrounding noise in its proximity, the downsizing of the magnetic sensor is required to raise a space resolution so that only a magnetic field produced by a magnetic field generating source to be targeted can be measured, however, there is a limit to the realization of downsizing of the magnetic field sensor from a structure viewpoint.
As a problem presented in an attempt to downsize the sensor, a reduction occurs in its characteristics caused by breaking of a wire of a central conductor of the semi-rigid coaxial cable constituting the magnetic field sensor or by deformation of an insulator. If it is difficult to downsize the magnetic sensor, due to problems related to self-inductance (which can be controlled by the downsizing of the magnetic sensor) or self resonance, the upper limit of a frequency band is confined to about 1 GHz. Accordingly, it is difficult to measure a magnetic field being induced by a clock signal having a high frequency component in the vicinity of, for example, LSI and MMIC.
Moreover, the attempt to downsize the shielded loop magnetic field sensor has not been made.
Also, the magnetic field sensor fabricated by using a conventional semi-rigid coaxial cable has a shortcoming that it is of low manufacturing accuracy and not suitable for volume production.